The tenth day of Muharram is considered by Shia Muslims as a day of mourning while Sunni Muslims consider it a day of joy and festivity. Based on several historical reports, the day of Ashura was described as a day of tragedy and mourning by the noble Prophet of Islam (PBUH) and his pure household (a.s).
Therefore, if we want to be genuine adherents of Islam and the loyal lovers of the holy Prophet and his pure household, we should regard the day of Ashura as a day of mourning, not a day of festivity.
The tenth day of Muharram, the first month in the Islamic calendar, is known as Ashura. It was the day Hazrat Husain son of Ali and the beloved grandson of Prophet Muhammad (s.a.w.a) was martyred. In the year 61 A.H, Imam Husain and seventy-two other members of his family and companions were mercilessly martyred by Yazid's army in Karbala.
Thenceforth, the day was marked and is commemorated annually among the Shia Muslims as a day of tragedy and mourning.
On the contrary, Yazid's army and the Umayyad dynasty became happy for thier apparent victory and celebrated the day as a day of joy and triumph. In order to cover up their genocide on the day of Ashura against the pure Household of the Prophet, the Umayyad dynasty fabricated various Hadiths to introduce the day as a day of joy and festivity[i]. From that event until present time, the day of Ashura is believed by Sunni Muslims to be a day of joy and festivity and that fasting is highly recommended there-in.
For the sake of argument, even if we assume that the day of Ashura was a day of joy and happiness for some selected Prophets of God as claimed by Sunni Muslims, but since on the same day the beloved grandson of the Prophet (s.a.w.a), i.e. Imam Husain along with seventy-two members of his family and companions (including a six-month old baby) were brutally killed by Yazid's army and their heads were cut-off from their bodies in Karbala, if we want to be genuine adherents of Islam and the loyal lovers of the holy Prophet (s.a.w.a) and his pure household, we should regard the day of Ashura as a day of mourning, not a day of festivity.
NOTE:
[i] There are several Hadiths which claim that Ashura is a day of joy and blessing, as Allah bestowed His blessings and favours on some Prophets or some communities. For instances, according to the aforementioned hadiths, the day of Ashura is indicated to be the same day Prophet Yunus was freed by the big fish; the same day Prophet Noah’s Ark safely landed; the same day children of Israel were liberated from Pharaoh's rule etc. But truth is, each of these events -based on reliable historical reports- occurred on different days and months, not on the tenth of Muharram.